Publications (21) View all
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Article: Nature of the band gap of In2O3 revealed by first-principles calculations and x-ray spectroscopy.
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ABSTRACT: Bulk and surface sensitive x-ray spectroscopic techniques are applied in tandem to show that the valence band edge for In2O3 is found significantly closer to the bottom of the conduction band than expected on the basis of the widely quoted bulk band gap of 3.75 eV. First-principles theory shows that the upper valence bands of In2O3 exhibit a small dispersion and the conduction band minimum is positioned at Gamma. However, direct optical transitions give a minimal dipole intensity until 0.8 eV below the valence band maximum. The results set an upper limit on the fundamental band gap of 2.9 eV.Physical Review Letters 04/2008; 100(16):167402. · 7.37 Impact Factor -
Article: Electronic structure of single-crystal rocksalt CdO studied by soft x-ray spectroscopies and ab initio calculations
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ABSTRACT: Soft x-ray emission spectroscopy (XES) and x-ray absorption spectroscopy (XAS) are employed to investigate the occupied and unoccupied electronic structures in rocksalt-phase single-crystal CdO. Resonant XES at the OK edge reveals a clear Cd 4d–O 2p hybridized peak and momentum-dependent coherent contributions to the resonant emission spectra. Good agreement is obtained between the above-threshold XES and XAS spectra, and the calculated O 2p local partial density of states (PDOS). Calculation of the O 2p PDOS was performed within the GW framework of many-body perturbation theory.Physical Review B 03/2008; 77:125204. · 3.69 Impact Factor -
Article: The band structure of WO3 and non-rigid-band behaviour in Na0.67WO3 derived from soft x-ray spectroscopy and density functional theory.
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ABSTRACT: The electronic structure of single-crystal WO3 and Na0.67WO3 (a sodium-tungsten bronze) has been measured using soft x-ray absorption and resonant soft x-ray emission oxygen K-edge spectroscopies. The spectral features show clear differences in energy and intensity between WO3 and Na0.67WO3. The x-ray emission spectrum of metallic Na0.67WO3 terminates in a distinct Fermi edge. The rigid-band model fails to explain the electronic structure of Na0.67WO3 in terms of a simple addition of electrons to the conduction band of WO3. Instead, Na bonding and Na 3s-O 2p hybridization need to be considered for the sodium-tungsten bronze, along with occupation of the bottom of the conduction band. Furthermore, the anisotropy in the band structure of monoclinic γ-WO3 revealed by the experimental spectra with orbital-resolved geometry is explained via density functional theory calculations. For γ-WO3 itself, good agreement is found between the experimental O K-edge spectra and the theoretical partial density of states of O 2p orbitals. Indirect and direct bandgaps of insulating WO3 are determined from extrapolating separations between spectral leading edges and accounting for the core-hole energy shift in the absorption process. The O 2p non-bonding states show upward band dispersion as a function of incident photon energy for both compounds, which is explained using the calculated band structure and experimental geometry.Journal of Physics Condensed Matter 04/2013; 25(16):165501. · 2.55 Impact Factor -
Article: Soft X-Ray Spectroscopic Study of Dense Strontium-Doped Lanthanum Manganite Cathodes for Solid Oxide Fuel Cell Applications
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ABSTRACT: The evolution of the Mn charge state, chemical composition, and electronic structure of (LSMO) cathodes during the catalytic activation of solid oxide fuel cell (SOFC) has been studies using X-ray spectroscopy of as-processed, exposed, and activated dense thin LSMO films. Comparison of O -edge and Mn -edge X-ray absorption spectra from the different stages of LSMO cathodes revealed that the largest change after the activation occurred in the Mn charge state with little change in the oxygen environment. Core-level X-ray photoemission spectroscopy and Mn resonant photoemission spectroscopy studies of exposed and as-processed LSMO determined that the SOFC environment ( ambient pressure of ) alone results in La deficiency (severest near the surface with Sr doping ) and a stronger contribution, leading to the increased insulating character of the cathode prior to activation. Meanwhile, O -edge X-ray absorption measurements support enrichment nearer the surface, along with the formation of mixed and/or passive and SrO species.Journal of The Electrochemical Society. 01/2011; 158(2):B99-B105. -
Article: First-principles calculation of resonant x-ray emission spectra applied to ZnO
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ABSTRACT: A framework for calculating the k-conserving component of K edge resonant x-ray emission spectroscopy measurements of anisotropic solids is presented. The crystalline band structure is calculated using a quasiparticle self-consistent GW implementation. Coherent spectra are calculated in the Kramers-Heisenberg formalism, and the effect of the experimental geometry in the dipole approximation is fully considered. Coherent spectra are calculated for ZnO and successfully compared to previously measured data.01/2011;
